Tom Smulders' favourites

Tom Smulders works at the Centre for Behaviour and Evolution and the Institute of Neuroscience at Newcastle University (UK). His research covers the interface between behavioural ecology and neuroscience.

"To someone trained in both evolutionary biology and neuroscience, the potential power of the comparative/evolutionary approach to understanding brain function is large. Unfortunately, the two approaches are not easily combined. Good comparative studies require large data sets from many species, while in-depth neuroscientific investigation is expensive and time-consuming on one species, let alone on many. The compromise often is to study easily-measured neuroscientific traits in this context. The most obvious one of those is total brain size (although investigations of sizes of specific brain areas have been performed as well). Total brain size is also interesting to many people because we humans, as a species, have an especially large brain for our body size. Understanding why we have this large brain and how it came about would give us interesting new insights into what it means to be human.

The studies I selected from Biology Letters are comparative studies on brain size that give us new insights into the selective pressures that have shaped total brain size. All these studies are about mammals, but there is no reason why their findings should not generalise to other vertebrate (or indeed invertebrate) taxa as well.

It is very likely that the evolution of brain size (and indeed more detailed anatomical and physiological features of brains) will be influenced by different selective pressures in different lineages and species. This selection of articles from Biology Letters gives us some flavour of what can be derived from relatively simple neuroscientific data (although usually analysed with quite sophisticated evolutionary techniques). I look forward to seeing more studies illuminating the mechanisms and patterns of brain evolution in Biology Letters."

Metabolic costs of brain size evolution

"In this article the authors show that species with relatively large brains also have relatively high basal metabolic rates (always controlling for total body size, which is highly correlated with both of these traits). This suggests that maintaining a larger brain requires more energy, even at rest, than maintaining a smaller brain."

Abstract:
In the ongoing discussion about brain evolution in vertebrates, the main interest has shifted from theories focusing on energy balance to theories proposing social or ecological benefits of enhanced intellect. With the availability of a wealth of new data on basal metabolic rate (BMR) and brain size and with the aid of reliable techniques of comparative analysis, we are able to show that in fact energetics is an issue in the maintenance of a relatively large brain, and that brain size is positively correlated with the BMR in mammals, controlling for body size effects. We conclude that attempts to explain brain size variation in different taxa must consider the ability to sustain the energy costs alongside cognitive benefits.

Bigger is not always better: when brains get smaller

"This article shows a nice example of how this energetic requirement can act as a selective force for smaller brains in bats. Those bats that can do with smaller brains, because they forage in the open, rather than in complex environments, have evolved smaller brains, relative to the inferred ancestral species. Those that need to navigate through tight spaces and complex environments, on the other hand, have increased brain sizes. The fact that energetic demands form a selective pressure that pushes brains to be smaller, means that for brains to become larger over evolutionary time, the selective advantages for bringing this about have to outweigh the energetic costs of maintaining these brains."

Abstract:
Many studies assume that an increase in brain size is beneficial. However, the costs of producing and maintaining a brain are high, and we argue that brain size should be secondarily reduced by natural selection whenever the costs outweigh the benefits. Our results confirm this by showing that brain size is subject to bidirectional selection. Relative to the ancestral state, brain size in bats has been reduced in fast flyers, while it has increased in manoeuvrable flyers adapted to flight in complex habitats. This study emphasizes that brain reduction and enlargement are equally important, and they should both be considered when investigating brain size evolution.

"This final paper shows that relative to their abundance in the population, larger-brained species are less preyed upon by predators (chimpanzees and felids) than smaller-brained species. This therefore suggests that one of the advantages of having a larger brain could be predator-avoidance."

Abstract:
Prey use a wide variety of anti-predator defence strategies, including morphological and chemical defences as well as behavioural traits (risk-modulated habitat use, changes in activity patterns, foraging decisions and group living). The critical test of how effective anti-predator strategies are is to relate them to relative indices of mortality across predators. Here, we compare biases in predator diet composition with prey characteristics and show that chimpanzee (Pan troglodytes) and felid show the strongest and the most consistent predator bias towards small-brained prey. We propose that large-brained prey are likely to be more effective at evading predators because they can effectively alter their behavioural responses to specific predator encounters. Thus, we provide evidence for the hypothesis that brain size evolution is potentially driven by selection for more sophisticated and behaviourally flexible anti-predator strategies.